Liquid scintillation counters are commonly used to measure the count rate or activity of samples containing low energy beta particles or corresponding particles emitting radionuclides such as tritium and carbon-14.
The range of the low energy beta particles in the sample is generally a few tens of micrometers at the most. As a consequence, the sample to be measured has to be placed in direct contact with a scintillation medium by dissolving or suspending the sample within the liquid scintillation medium in a container so that the emitted beta particles can interact with the molecules of the liquid scintillation medium. The medium comprises a solvent or solvents and a solute or solutes present in a few percent by weight of the solution. In this interaction process, most of the kinetic energy of the interacted beta particle is absorbed by the solvent and then transferred to the solute which emits scintillation photons in an amount proportional to the energy of the interacted beta particle. These scintillation photons are detected by two photomultiplier tubes simultaneously producing electric pulses. The sum pulse height is proportional to the energy of the interacted beta particle.
The conventional liquid scintillation counters have been designed to measure samples which are in sample vials with the scintillation medium. The volume of the sample vial is typically 6 or 20 milliliters. The sample vials are placed in elongated sample holders, which have a plurality of compartments for individual sample vials. A plurality of types of sample vial holders may include holders characterized by the size of the sample vials they are adapted to contain. The sample vial holders are placed on the conveyor of the automatic sample changer system of the liquid scintillation counter.
Because the conventional liquid scintillation counters have been designed to measure samples in vials, whose volumes are up to 20 milliliters, difficulties will be encountered when the sample volumes are only a few hundred microliters or less. Typically, these kinds of samples with medical or biological interest are prepared in small tubes, which have been inserted into normal vials. In addition, the handling of separate sample vials is very time consuming and includes a potential risk of setting the sample vials in incorrect order into the sample holders. The sample changing mechanism of such an instrument is also rather complicated, because the vial must be removed from the sample plate holder and must be positioned into a light tight radiation detection chamber and, after counting, it must be returned back to the sample holder.
In addition to the above mentioned conventional liquid scintillation counters, one special purpose liquid scintillation counter exists, identified by the trade name 1205 Betaplate, and manufactured by Wallac Oy, Finland. Originally this instrument was designed to measure liquid scintillation samples deposited on a filter mat and sealed into a plastic bag with a small amount of scintillation liquid. The maximum diameter of these sample spots is 10 millimeters and the distance between the center points is 15 millimeters. A total of 96 sample spots are arranged in a 6.times.16 matrix format. The Betaplate is provided with 6 detectors, one for each row, thus 6 samples will always be measured simultaneously.
The Betaplate has been developed to measure samples deposited in a type of multi-well sample plate comprising 96 separate sample wells arranged in the same 6.times.16 format and with the same 15 millimeter distance between the center points as in the filter mats. The maximum height of this multi-well sample plate is 7.5 millimeters and the maximum volume of the sample well is limited to 400 microliters.
Thus the Betaplate is applicable for measuring 6 samples simultaneously deposited on filter mats or multi-well sample plates having only one format. Unfortunately, in practice, there is a need for multi-well sample plates with various formats and well volumes. In addition, a six detector liquid scintillation counter will be too expensive in some applications due to the relatively small amount of samples to be measured.